Chain Extenders for PET and rPET: 5 Reactive Types, Dosage and Selection

Chain extenders for PET are multifunctional reactive additives, mainly epoxide oligomers and dianhydrides, that are fed into the melt at 0.2 to 1.5 wt% to recouple the shortened chains of recycled PET and rebuild its intrinsic viscosity. Recycled PET needs them because every melt pass and every trace of moisture cuts its chains and lowers its IV; so which chemistry repairs which defect, and how much is too much?

Chain extenders sit in the reactive corner of the plastic additives toolbox: they change the polymer itself rather than sitting between the chains.

This guide compares the 5 reactive chemistries used in PET and rPET, sets the dosage window by application, marks the gel limit, explains reactive extrusion, lists the IV and rheology tests that confirm the reaction, separates what EU and US food-contact law allow in recycled PET from what remains unconfirmed, and names the suppliers of each chemistry.

In numbers:

• 0.5 to 1.5 wt% multifunctional epoxide oligomer in rPET holds gel content at or below 2 %
• About 32 % gel forms at 3 wt% Joncryl ADR-4400 in rPET (Karl et al., 2024)
• 0.2 to 0.3 wt% PMDA is the level used in PET strain-hardening and extensional-rheology studies
• 30 % post-consumer recycled content is required in contact-sensitive PET packaging from 1 January 2030 (Regulation (EU) 2025/40, Article 7)

Why Does PET Need Chain Extenders?#

PET needs chain extenders because it is a condensation polymer whose chains break every time it is melted with residual moisture, and each break leaves a new carboxyl end group that catalyses the next break. PET is a step-growth polymer processed at 240 to 320°C, so its chain ends, not its backbone, decide how it flows, foams and blow-moulds, and a shortened chain lowers molar mass and intrinsic viscosity together. The hub on chain extenders for polymers compares the same chemistries across PET, PLA, PBT and polyamide.

How does PET lose intrinsic viscosity during recycling?#

Recycled PET loses intrinsic viscosity through hydrolysis in the melt: water left in the flake splits an ester bond, the split creates one hydroxyl and one carboxyl end group, and the new acid end group catalyses the next hydrolysis step. Thermal and thermo-oxidative chain scission add to this loss above 300°C, and PVC or residual moisture contamination in the incoming rPET stream raises the rate further (Jang et al., 2022, polymer-stability review).

Hydrolysis is one of four routes of polymer degradation that act on polyester.

IV loss in rPET runs in 4 steps:

1. Residual moisture in the flake reaches the melt at 240 to 320°C.
2. Water attacks an ester linkage and splits the chain.
3. The split leaves one hydroxyl and one new carboxyl end group on the two fragments.
4. The new carboxyl end group catalyses the next hydrolysis step, so IV keeps falling with every pass.

The IV a converter needs depends on the product: a textile fibre grade starts at 0.40 to 0.70 dL/g while a carbonated soft-drink bottle needs 0.78 to 0.85 dL/g, so the same rPET flake can be fit for one product and unfit for the other.

What does a chain extender do in PET?#

A chain extender is a small multifunctional molecule that reacts with the carboxyl or hydroxyl end groups of PET and joins two or more chains into one, which raises molar mass, melt viscosity and melt strength in a single extrusion pass. A bifunctional molecule, such as a diisocyanate or a bisoxazoline, can only rejoin two chain ends into a longer linear chain, while a multifunctional epoxide or a dianhydride carries several reactive groups and can pull several PET chains onto a single branch point.

Functionality decides the architecture: a bifunctional extender gives linear chains, while a tetrafunctional or higher one gives star and comb structures, and only branched structures give the strain hardening that foaming and thermoforming need. Long-chain branching (LCB) therefore matters more than molar mass alone whenever the downstream process stretches the melt, as extrusion foaming, blow moulding, film blowing and thermoforming all do.

Is a chain extender the same as a hydrolysis stabilizer?#

No: a chain extender rebuilds molar mass that has already been lost, while a hydrolysis stabilizer removes the acid end groups that cause the next loss, and carbodiimides are the one chemistry that does both. Its N=C=N group reacts with a carboxyl end group to form an N-acylurea, removing the catalytic acid and coupling two chains in the same step. The full class of hydrolysis stabilizers is compared by polymer on its own page.

What Are the 5 Types of Chain Extenders for PET?#

The 5 types of chain extenders used in PET are multifunctional epoxides, dianhydrides, bisoxazolines, carbodiimides and diisocyanates, and multifunctional epoxides carry almost all of the industrial rPET volume. Each chemistry attacks a different end group, at a different functionality, and builds a different chain architecture, from linear extension to star and comb long-chain branching. Table 1 sets out the reactive group, end group, functionality, architecture and by-product for all 5, in the order used throughout this page.

Table 1. Reactivity matrix for the 5 chain extender chemistries in PET

1. Multifunctional epoxides (styrene-acrylic glycidyl oligomers)#

Multifunctional epoxides are styrene-acrylic oligomers that carry several pendant glycidyl groups, each of which opens on contact with a carboxyl end group of PET and links that chain to the oligomer backbone. The glycidyl group reacts quickly with carboxyl end groups and more slowly with hydroxyl end groups, so in carboxyl-rich degraded rPET it couples several PET chains onto one oligomer, building star and comb long-chain-branched structures that raise molar mass, melt viscosity, melt strength and strain hardening in a single pass.

The oligomer reacts first with low-molar-mass contaminants and short oligomeric fragments in the melt, and only once those are consumed does it start coupling the PET backbone, which is why the same nominal dosage behaves differently on flake with a different contamination history. Overdosing, or holding the reaction too long in the extruder, produces gels rather than more useful branching.

Two older epoxy chemistries share this family history: TGIC (functionality 3) has been on the REACH SVHC Candidate List as a mutagen since 18 June 2012 and is mostly replaced, and TGDDM (functionality 4) is cited as a branching agent; neither has a substance page here, so both stay as plain text. The chemistry of epoxy chain extenders is covered grade by grade on its own page.

Joncryl ADR-4368 vs ADR-4400 vs ADR-4468#

Joncryl ADR-4368 carries about 9 epoxy groups per molecule at an epoxy equivalent weight of 285 g/mol and a molar mass of 6,800 g/mol, while ADR-4400 has a lower number-average functionality of about 5 at a similar molar mass of 7,100 g/mol. ADR-4468 sits at the high end of the range, with a number-average functionality of about 9 and a weight-average functionality of about 24. A glass transition temperature of 54°C and a density of 1.08 g/cm3 have been reported for ADR-4368(-S), but from a PLA/PA11 study rather than a PET trial.

Higher functionality builds branching faster at the same weight percentage, and it also reaches the gel point sooner, so the grade and the dosage are one decision, not two. Joncryl ADR grade data, applications and regulatory status sit on the substance page.

Joncryl ADR grades

2. Dianhydrides (PMDA)#

Pyromellitic dianhydride (PMDA, CAS 89-32-7) opens its two anhydride rings against the hydroxyl end groups of PET, forming ester links and four reactive arms, which builds star-shaped and hyperbranched PET without releasing any volatile by-product. PMDA is C10H2O6, EC 201-898-9, with a molar mass of 218.12 g/mol, a melting point of 286 to 287°C and a density of 1.68 g/cm3. Its tetrafunctionality is what gives the branching, and because it is thermally stable, it needs no volatile by-product route to react.

PMDA must be kept dry: it absorbs water, and the hydrolysed acid form promotes degradation instead of repairing it. Its reactivity towards hydroxyl end groups is comparatively low, which is one reason it suits a different end-group balance than the epoxides; PET and rPET extensional-rheology studies use it at 0.2 to 0.3 wt%. Pyromellitic dianhydride (PMDA) is also a polyimide monomer, which is where its thermal stability comes from.

3. Bisoxazolines (1,3-PBO)#

Bisoxazoline (1,3-PBO, CAS 34052-90-9) couples PET chains end to end: each of its two oxazoline rings opens against a carboxyl end group and forms an ester-amide link, which raises molar mass and lowers the acid number at the same time. It is 2,2'-(1,3-phenylene)bis(2-oxazoline), EC 421-510-3, C12H12N2O2, molar mass 216.24 g/mol, a dry powder. Because the molecule is bifunctional, coupling stays linear rather than branched, and the reaction leaves no by-product, so it is often combined with an anhydride or a diisocyanate.

No PET dosage for 1,3-PBO is published in our sources; converters set the level by trial. Identity and regulatory data for bisoxazoline are on the substance page.

4. Carbodiimides (monomeric and polymeric)#

Carbodiimides sit between the two jobs: the N=C=N group reacts with a carboxyl end group of PET to form an N-acylurea, which both removes the acid that drives hydrolysis and couples two chains. The monomeric grade is bis(2,6-diisopropylphenyl)carbodiimide, CAS 2162-74-5, EC 218-487-5, C25H34N2, 362.5 g/mol, sold as Stabaxol I (LANXESS) and Stabilisator 7000 (Raschig). Polymeric grades, Stabaxol P, P LF, P 100, P 110 and P 200 (LANXESS) and Carbodilite HMV-15CA and HMV-5CA-LC (Nisshinbo), extend the same chemistry into a longer chain.

Finished polyester products take about 0.5 to 2.5 parts per 100 of a polycarbodiimide; the PET masterbatch route runs at 10 to 20 parts of masterbatch per 100 PET, which is 1.5 to 3.0 wt% active with a 15 % concentrate such as Stabaxol KE 7646 or MB PET 3040. A typical use is PET monofilament for paper-machine dryer screens at about 100°C in water, where the service-life gain claimed for Stabaxol is usually threefold. Converters should treat carbodiimides as hydrolysis stabilizers that extend chains, not as a substitute for an epoxide when a large IV gain is required. Monomeric and polymeric grades are separated on carbodiimide hydrolysis stabilizers, and polycarbodiimide grades carry the masterbatch route into PET.

5. Diisocyanates#

Diisocyanates couple PET chains through their NCO groups, which react with both hydroxyl and carboxyl end groups, but they are the least used of the 5 chemistries in polyester recycling because of their handling and sensitisation profile. Because the NCO group is bifunctional, coupling stays linear. In EU food contact, hexamethylene diisocyanate is listed as FCM 372 with a limit of 1 mg/kg in the final product expressed as isocyanate moiety. The short-chain diols and diamines sold as polyurethane chain extenders are PU raw materials, not polyester additives, and are not covered here.

Epoxy Chain Extender vs PMDA in rPET: Which Works Better?#

Multifunctional epoxides work better than PMDA in recycled PET because degraded rPET is carboxyl-rich, and the glycidyl group reacts with carboxyl end groups far faster than an anhydride ring reacts with hydroxyl end groups. C.W. Karl and co-workers at SINTEF and the University of Hannover reported in Industrial & Engineering Chemistry Research in 2024 that compounding rPET with Joncryl ADR-4400 at 0.5, 1.5 and 3 wt%, or with PMDA at 3 wt%, in a mini-extruder at 280°C gave the epoxide larger increases in inherent viscosity and extrusion force than PMDA achieved.

The end-group balance decides the winner. Epoxides attack carboxyl end groups, which dominate a degraded rPET flake; PMDA attacks hydroxyl end groups, which are scarcer, and its reactivity towards them is low to begin with. PMDA also under-performs in wet flake, since it is hygroscopic and its hydrolysed acid form promotes degradation instead of repairing it. TGIC has carried an SVHC listing since 18 June 2012 and is largely out of use, and diisocyanates are ruled out in many operations by their worker-sensitisation profile.

Table 2 sets out the 7 criteria that separate the two chemistries in rPET.

Table 2. Multifunctional epoxide vs PMDA in rPET

How Much Chain Extender Does rPET Need?#

Recycled PET takes 0.5 to 1.5 wt% of a multifunctional epoxide or 0.2 to 0.3 wt% of PMDA, set by the incoming IV, the contamination level of the flake, the melt-strength target and the gel tolerance of the product. Table 3 gives the published dosage for every chemistry on this page.

Four factors drive the dosage inside those windows:

• Incoming IV and carboxyl end-group count of the flake
• The share of low-molar-mass contaminants that consume the extender before the PET chains do
• The melt strength the downstream process needs, with foam and thermoforming needing the most
• The gel tolerance of the product, with film and fibre tolerating the least

Table 3. Master dosage table

These are published study and supplier ranges for the grades named. The incoming flake decides the final level, and every change of recyclate source needs its own trial. Dosages are not transferable between grades of different functionality.

Dosage by application: sheet, strapping, fibre, foam and bottle#

The dosage a converter needs follows the process, not the resin grade: thermoformed sheet and extrusion foam need the branching that sits at the top of the 0.5 to 1.5 wt% epoxide window, while fibre and thin film sit at the bottom of it because a single gel particle breaks a filament. This split is a rule derived from the gel limit and the melt-strength requirement of each process, not a published per-application dosage table.

The 5 applications position differently inside the dosage window:

• Extrusion foam and thermoformed sheet, needing the most long-chain branching, at the top of the window
• Strapping, needing melt strength for the draw step, in the upper half
• Blow-moulded bottle preforms, needing moderate branching without excess gel risk
• General film, sitting lower in the window to limit gel-related defects
• Fibre and monofilament, at the bottom of the window because a single gel particle breaks a filament

Extrusion foam draws on additives for plastic foams beyond the chain extender alone, and fibre and bottle applications each carry their own additive set too.

The gel limit: what happens when rPET is overdosed#

Gels form when a multifunctional epoxide keeps reacting past the point where branches start to join into a network, and the transition is sharp: 1.5 wt% of Joncryl ADR-4400 in rPET left 2 % or less gel, while 3 wt% produced about 32 %. These figures come from Karl and co-workers' 2024 mini-extruder trials at 280°C, and they describe that equipment and that study, not a production specification for every line.

An excessively long residence time has the same effect as overdosing: reactive sites that would otherwise stop at a star or comb structure keep finding new partners until the branches cross-link into a gel network. Overdosed chain extender is one named cause of gels and fisheyes in plastic film, and film and fibre lines, which tolerate the least gel of any application, sit deliberately at the bottom of the dosage window for this reason.

Chain extender masterbatch and let-down ratio#

Most converters dose a chain extender as masterbatch rather than as powder, because a pellet feeds accurately at 0.5 to 1.5 wt% and does not dust or segregate in the throat of the extruder. CESA-Extend, Avient's chain-extender masterbatch line built from the Clariant masterbatch business Avient acquired in 2020, and Stabaxol MB PET 3040, a 15 % active concentrate from LANXESS, both follow this route.

A worked example shows the arithmetic. A masterbatch that carries 15 wt% active chain extender, let down at 10 parts per 100 parts PET, gives 1.5 wt% active chain extender in the finished compound on the simple parts-per-hundred convention (0.15 × 10 parts masterbatch ÷ 110 total parts × 100 on the strict total-weight basis). Check the arithmetic, and see how the basis changes the result, in the let-down ratio calculator. This is a calculation example, not a supplier recommendation for any specific line.

How Is a Chain Extender Added to PET in the Extruder?#

A chain extender is added by reactive extrusion: the dried extender or its masterbatch is metered into a dried PET melt at 240 to 320°C, and the reaction completes inside the residence time of the screw. Moisture and residence time together decide whether the reaction stops at useful branching or runs on to a gel. A published laboratory protocol dries rPET flake at 140°C for 17 hours and Joncryl at 120°C for 3 hours before compounding; PMDA needs the same discipline because it too is hygroscopic.

Reactive extrusion of a chain extender in PET follows 5 steps:

1. Dry the flake and the additive before compounding.
2. Meter the extender or masterbatch gravimetrically.
3. Hold the melt above the activation temperature long enough for conversion.
4. Keep the residence time short enough to stay below the gel point.
5. Check IV, MVR and gel content on the pellet before the product run.

In a polyamide 10.10 study, both Joncryl grades reached 99 % conversion at 200°C in 120 seconds or at 280°C in 30 seconds; the supplier recommends about 260°C with at least 120 seconds of dwell for PET, though the conversion figure itself is from the polyamide study. Reactive extrusion is one mode of plastic compounding.

How Is Chain Extension in rPET Measured?#

Chain extension in rPET is measured on 4 properties: intrinsic viscosity, melt volume rate, melt strength and gel content, with intrinsic viscosity as the number that the buyer specifies. Each property tracks a different part of the same reaction: IV and melt strength track how much the chains have grown and branched, while gel content tracks whether the reaction went too far.

Intrinsic viscosity (ASTM D4603, ISO 1628-5)#

Intrinsic viscosity is measured to ASTM D4603 or ISO 1628-5 by dissolving the PET in a solvent and comparing the flow time of the solution with that of the pure solvent, and a chain extender raises it by rejoining the chains that recycling cut. The method reports IV in dL/g, and the intrinsic viscosity of PET and recycled PET page sets out the full method and its solvent systems.

No published before-and-after IV figure for a chain-extended rPET exists in our sources, so this page states the direction of the change and points to the studies rather than quoting a gain. Table 4 gives the IV band each PET product needs, which shows which products a given rPET flake can still serve even before any chain extender is added.

Table 4. Intrinsic viscosity by PET product

Grade bands, not chain-extension results. They show which products a given rPET flake can still serve.

Melt strength, MVR and extensional rheology#

Melt volume rate to ISO 1133 falls as the chain extender works, because longer and branched chains flow more slowly under the same load, which makes MVR the fastest shop-floor check of a chain-extension step. Melt flow rate and melt volume rate follow ISO 1133 and ASTM D1238, capillary rheometry follows ISO 11443, and melt strength is read on a Rheotens-type instrument that pulls the melt strand down at increasing speed until it breaks.

Three methods together confirm a chain-extension step:

• Melt volume rate or melt flow rate to ISO 1133 or ASTM D1238, checked against the pre-reaction pellet
• Capillary rheometry to ISO 11443, for a fuller viscosity curve across shear rates
• Extensional rheology, which reads the strain hardening that PMDA studies use to set the 0.2 to 0.3 wt% dosage window

A falling melt flow rate signals that the chains have been rejoined.

Gel content, colour and carboxyl end groups#

The 3 checks that catch a failed chain-extension step are gel content, yellowness index to ASTM E313 and the carboxyl end-group count.

• Gel content, measured by solvent extraction of the insoluble fraction, is the overdose indicator described in the gel-limit section above; the full method is on gel content.
• Yellowness index to ASTM E313 catches the thermal yellowing that the same heat history driving the reaction also drives, and never the withdrawn ASTM D1925.
• Carboxyl end-group titration tracks the route bisoxazoline uses, because it lowers the acid number as it couples chains.

No pass or fail limit for rPET is published in our sources; the end point is set by the product specification.

Which Chain Extenders Are Allowed in Food-Contact rPET?#

Food-contact recycled PET may contain only chain extenders that are themselves listed in Regulation (EU) No 10/2011, because Regulation (EU) 2022/1616 does not create a separate positive list for additives used inside a recycling process. The same principle applies whichever chemistry is used: the listing status of the substance itself, not the recycling process around it, decides whether it may stay in food-contact rPET. The wider set of rules is on additives for food packaging.

EU 10/2011 and Regulation (EU) 2022/1616#

Pyromellitic dianhydride illustrates the route: it is authorised in the EU as FCM 166, listed under the name pyromellitic anhydride as a monomer or starting substance with a specific migration limit of 0.05 mg/kg, and that listing is what allows it in food-contact recycled PET. Bis(2,6-diisopropylphenyl)carbodiimide is listed the same way, as FCM 438 (Ref 13303), monomer or starting substance only, SML 0.05 mg/kg as the sum of the carbodiimide and its hydrolysis product 2,6-diisopropylaniline. Hexamethylene diisocyanate is FCM 372, limited to 1 mg/kg as isocyanate moiety. Bisoxazoline (1,3-PBO) is not listed in EU 10/2011's Annex I, consolidated 16 March 2025.

Joncryl ADR's EU food-contact status is not confirmed in our sources. A related copolymer, CAS 37953-21-2, is FCM 857, an additive restricted to rigid PVC at a maximum of 2 % at room temperature or below, and whether any Joncryl grade maps to that entry is unverified; its glycidyl methacrylate monomer is FCM 220, SML 0.02 mg/kg. Decontamination, the public register and the FDA route are compared on recycled plastics regulations.

Table 5. Regulatory matrix for PET chain extenders

An FCM listing for a monomer or starting substance is not a general clearance. Check the restriction wording and the food type in the consolidated Annex I text before using it.

The PPWR recycled-content targets behind rPET demand#

The commercial case for chain extenders in PET is written into Article 7 of the Packaging and Packaging Waste Regulation, Regulation (EU) 2025/40, which requires 30 % post-consumer recycled content in contact-sensitive PET packaging from 1 January 2030 and 50 % from 2040. Regulation (EU) 2025/40 was published in the Official Journal on 22 January 2025 and applies from 12 August 2026. Article 7 sets separate targets for other contact-sensitive packaging, single-use plastic beverage bottles and other plastic packaging, each rising between the 2030 and 2040 milestones, which is the direct commercial link between PPWR and this page.

United States: the FDA recycled-plastics route#

In the United States, a recycling process for food-contact PET is assessed against a negligible dietary concentration of 0.5 ppb, and any chain extender left in the resin is judged as part of that assessment rather than under a separate positive list. No FDA Food Contact Notification status for Joncryl ADR is recorded in our sources. Both systems, EU or US, are verified by migration testing on the finished article.

Who Supplies Chain Extenders for PET and rPET?#

Chain extenders for PET and rPET come from BASF (Joncryl ADR), Avient (CESA-Extend masterbatch), Sukano, Nexam Chemical, LANXESS and Nisshinbo (carbodiimides), and the same chemistry is sold both as a neat oligomer and as a masterbatch. The Joncryl ADR line originated at Johnson Polymer before BASF acquired it; Avient built CESA-Extend from the Clariant masterbatch business it acquired in 2020 for about USD 1.6 billion, having itself been formed as PolyOne on 31 August 2000. LANXESS supplies the Stabaxol carbodiimides and Nisshinbo the Carbodilite grades.

Buyers should compare grades by reactive functionality and epoxy equivalent weight, not by trade name, since two oligomers with the same active content reach the gel point at different dosages. PMDA producers are recorded in our dossier as Lonza and Daicel, not yet verified against company sources.

Table 6. Chemistry, brand and supplier

Not a performance-equivalence claim.

The full BASF plastic additives portfolio lists Joncryl next to its stabilizer brands, CESA-Extend is one line in the Avient plastic additives portfolio, and Stabaxol sits in the Lanxess plastic additives portfolio. Send one request to several chain-extender suppliers with the plastic additive supplier finder.

─── The sections below step outside PET chain extenders into the wider rPET additive package and the same chemistry in other polymers. ───

What Other Additives Does Recycled PET Need?#

Recycled PET needs a full additive package in which the chain extender is only the reactive layer, next to antioxidants and hydrolysis stabilizers that stop the next round of degradation, acetaldehyde scavengers, reheat additives, toners and oxygen barriers. The complete package also includes polycondensation catalysts based on antimony, titanium, germanium or aluminium, nucleating agents for CPET, slip and antiblock additives for film, antistatics, optical brighteners and delustrants, none of which the chain extender replaces. The complete package is on additives for PET resin.

Restabilization across all polymers is on additives for recycled plastics, and mixed recyclate streams need compatibilizers for recycled plastics rather than a chain extender when the problem is two incompatible polymers rather than one shortened chain.

Hydrolysis stabilizers and antioxidants for rPET#

Chain extension repairs the damage that recycling has already done, while restabilization prevents the next round: a carbodiimide removes the acid end groups, and an antioxidant package protects the melt during the pass that follows. Restabilization of recyclate is an established compounding practice, associated in the literature with R. Pfaendner's work on antioxidant packages for post-consumer resin. The restabilization dosages in our source library are for recycled polypropylene and HDPE, not for rPET, so they are not repeated here. Restabilization of recycled plastics covers the antioxidant side of the same problem.

Chain extenders for PLA, PBT and polyamide compared#

The same epoxide oligomer serves PLA, PBT and polyamide, but the reason changes: in PET it repairs intrinsic viscosity, in PLA it also builds the melt strength that foaming needs, and in polyamide blends it works as a reactive compatibilizer at the interface. Chain extenders for PLA carry the foaming case in full; this page gives no PLA dosage figure because our source library carries two conflicting ranges for that polymer.

Three polymers share the epoxide chemistry with PET:

• PLA, where the same oligomer builds foaming melt strength alongside IV repair
• PBT, which uses the same carbodiimides for hydrolysis protection; see additives for PBT
• Polyamide, where the oligomer works as a reactive compatibilizer at the blend interface

Polycarbodiimides also extend into PBS, PHA, TPU, TPE-E and EVA.

Can a chain extender replace solid-state polycondensation?#

A chain extender and solid-state polycondensation both raise the molar mass of PET, but they are not interchangeable: SSP builds molar mass slowly in the solid phase over hours, while a chain extender does it inside the residence time of one extruder pass. SSP is one alternative listed alongside PMDA in the literature; no comparative IV, cost or throughput data exist in our sources, so the difference is described here only in qualitative terms.

Does a chain extender change the colour of rPET?#

Colour is measured, not assumed: yellowness index to ASTM E313 is part of the release test for chain-extended rPET, because the same heat history that drives the reaction also drives thermal yellowing. No colour-improvement claim for any named chain extender grade is stated on this page, because none is established in our sources; the test method, not a marketing claim, is the answer.

Is chain-extended rPET still recyclable?#

Chain-extended rPET stays mechanically recyclable as PET, because the extender is present at well under 2 wt% and the branches it creates are still polyester, but gels formed by overdosing carry into the next loop as unmelted particles. At the 0.5 to 1.5 wt% dosage window that keeps gel content at or below 2 %, the resin reprocesses normally; the roughly 32 % gel level seen at 3 wt% would not. Design for recycling sets out which additives survive a second loop.

Are chain extenders hazardous?#

PMDA is the one chain extender for PET with a harmonised EU hazard classification: under CLP Annex VI index 607-098-00-X it is classified Eye Dam. 1 (H318), Resp. Sens. 1 (H334) and Skin Sens. 1 (H317), and occupational asthma has been reported. Bisoxazoline and the monomeric carbodiimide carry no harmonised CLP entry; the carbodiimide's supplier self-classification (H302, H360(F), H372, H373) is a manufacturer's own classification, not a harmonised one. The Joncryl oligomer is not listed under Proposition 65, though residual glycidyl methacrylate was listed as a carcinogen on 27 January 2023.